flattening on Binary Tree
Last updated: October 1, 2025
Quick Overview
Given a binary tree, flatten it into a linked list in-place following the same order as a pre-order traversal. The output should be the head of the newly formed linked list, where each node's left child is null and the right child points to the next node in the list.
Slack
October 1, 20256
10
130 solved
Given a binary tree, flatten it into a linked list in-place following the same order as a pre-order traversal. The output should be the head of the newly formed linked list, where each node's left child is null and the right child points to the next node in the list.
Slack uses this problem in the Phone Screen to evaluate your algorithmic thinking. They expect you to discuss multiple approaches, analyze trade-offs between them, and implement the optimal solution with clean, readable code.
What the Interviewer Expects
- Identify the correct data structure and algorithm for the problem
- Write clean, bug-free code with proper variable naming
- Analyze time and space complexity correctly
- Handle basic edge cases (empty input, single element)
- Communicate your thought process while coding
Key Topics to Cover
How to Approach This
- Clarify input constraints and edge cases before writing code.
- Walk through your approach verbally and confirm with the interviewer before coding.
- Start with a brute force solution, then optimize. Mention time and space complexity.
- Test your solution with examples, including edge cases like empty input or duplicates.
- Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
- How would your solution change if the input was sorted?
- What happens if the input contains duplicates?
- How would you test this solution thoroughly?
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Practice DSA ProblemsSample Answer
Problem Analysis
To flatten the binary tree into a linked list following the pre-order traversal, we can identify that a depth-first search (DFS) approach is most suitable here. The pattern we will use is recursive tr...
Approach
- Start at the root of the binary tree.
- Use a recursive helper function that takes the current node as input.
- If the current node is null, return.
- Preserve the left and right children of ...
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